1. Field of the Invention
The present invention relates to a position sensor for detecting a position of an object, and more particularly to a position sensor having one or more magnetic resistance effect devices (hereinafter referred to as a "MR device"), which detects a position for an object with high precision and resolution.
2. Brief Description of the Related Art
A lens system having a zoom function and a focus function is an example of a device which requires detecting a position with high resolution. In the lens system, the distance to the subject and the position of the zoom lens and the focus lens must be at the predetermined position which the lens design determines. In order to determine the predetermined position, a method of using a mechanical cam or a method of using an electronic cam which detects the position of the zoom lens and focus lens and controls them by computing has been used in recent years.
However, the method of using a mechanical cam has problems, such as making the device with a large dimension and degrading the precision due to mechanical wear.
Methods of using an electronic cam include a method of using a potentiometer as the position detector, and a method of counting control signals of a stepping motor which is used as the driving motor for the lens. In the first method mentioned above, there are problems such that the load applied on the motor is increased since the method is of a contact, the linearity is worsened due to dispersion of the resistance film, and reliability is unsatisfactory. In the case of the second method using a stepping motor, a lens system using a stepping motor is merely applicable, and errors can be caused if there is an object between the motor and the lens, such as a gear due to a play existing therebetween.
In order to overcome the aforementioned problems, there can be a method of using a MR device for position detection, utilizing the property that the resistance value of the MR device varies in response to a change of a magnetic field. There is, for instance, a method in which a magnet is disposed on the lens side, a MR device is installed, and the lens together with the magnet is moved with respect to the MR device, thus the resistance variation is produced as the position variation.
The conventional methods of position detection using MR devices are described with reference to the following two examples.
FIG. 13 illustrates a pattern formed of a MR device utilized in the first example. The MR device includes two magnetic sensing elements M71 and M72, which are formed of a ferromagnetic thin film of, for example, nickel-iron alloy deposited on a glass substrate by vacuum evaporation or sputtering, and thereafter formed two parallel thin strips that meet at a folded point 71 by etching or the like process. The folded point 71 is formed thicker to reduce the resistance value. Magnetic sensing elements M71 and M72 are formed as elongated in the direction perpendicular to the magnetic field of the scale structure 7 in order to cause resistance change in response to the field. A connecting lead 72 and a wiring lead 73, for connecting the sensing elements M71 and M72 as well as for connecting the elements M71 and M72 with external connecting terminals, which will be hereinafter described, are formed of the same ferromagnetic thin film as the sensing elements M71 and M72. The connecting lead 72 is formed in the direction along which a resistance change would not be caused, namely in the same direction as that of the magnetic field of scale structure 7. The connecting lead 72 and wiring lead 73 are thickened in order to lower the connecting resistance. External connecting terminals include those 74 and 76 for applying a source voltage for the MR device, and a terminal 75 for producing an output signal. These terminals 74, 75 and 76 are also formed of ferromagnetic thin film same as the magnetic sensing elements M71 and M72. The scale structure 7 includes N- and S-poles, each being formed alternatively in the opposite polarity with a predetermined distance P as shown in FIG. 13.
FIG. 14 shows an equivalent circuit to the MR device of this first embodiment, and FIG. 15 shows the relationship between the sensing elements M71 and M72 of the MR device and an output signal of the MR device. The sensing elements M71 and M72 are formed with a distance of p/2 apart so as to generate a sine wave output signal.
FIG. 16 illustrates a pattern formed of a MR device utilized in the second example. The MR device includes four magnetic sensing elements M81, M82, M83 and M84, which are formed of a ferromagnetic thin film of, for example, nickel-iron alloy deposited on a glass substrate by vacuum evaporation or sputtering, and thereafter formed two parallel thin strips by etching or the like process. A folded point 81 is formed thicker to reduce the resistance value. Magnetic sensing elements M81 to M84 are formed as elongated in the direction perpendicular to the magnetic field of the scale structure 7 in order to cause resistance change in response to the field. Each connecting lead 82 for connecting the sensing elements M81 to M84 is formed of the same ferromagnetic thin film as the sensing elements M81 to M84. The connecting lead 82 is formed in the direction along which a resistance change would not be caused, namely in the same direction as that of the magnetic field of scale structure 7. The connecting lead 82 and wiring lead 83 are thickened in order to lower the connecting resistance. External connecting terminals include those 84 and 86 for applying a source voltage for the MR device, and a terminal 85 for producing an output signal. These terminals 84, 85 and 86 are also formed of ferromagnetic thin film same as the magnetic sensing elements M81 and M82. The following connections including: the magnetic sensing element M84 with the terminal 86; and the connecting lead 82 between the elements M82 and M83 with the terminal 85; are formed by each associated wiring lead 83 respectively. The scale structure 7 includes N- and S-poles, each being formed alternatively in the opposite polarity with a predetermined distance P.
FIG. 17 shows an equivalent circuit to the MR device of this second example, and FIG. 18 shows the relationship between the sensing elements M81 to M84 of the MR device and an output signal of the MR device. For the purpose to eliminate the error produced in the boundary region between an N-pole and an S-pole adjacent to each other, the magnetic sensing elements M81 and M82 are formed away from each other by the same distance as the pole width P, and, also the magnetic sensing elements M83 and M84 are formed in the same manner. In order to produce a sine output signal, the one magnetic sensing group, comprising the elements M81 and M82, and the other sensing group, comprising the elements M83 and M84, are formed. With a distance of p/2 apart so as to generate a sine wave output signal.
The output signals are converted to pulse signals, whose edges are measured to obtain the position 10 information, and, by interpolation the output signal curve in the analog form, a position information with further increased resolution can be obtained.
In the example above, the resistance variation can be caused by the magnetic field in either of magnetic sensing elements M71, M72 and M81 to M84, connecting terminals 74, to 76 and 84 to 86 or wiring leads 73 and 83.
In particular, since the wiring leads 73 and 83 are formed in the direction orthogonal to the magnetic field of scale structure 7, the resistance variation at this region is so large that the correct position information cannot be obtained.
The MR device can exhibit the resistance change of different hysteresis characteristics between the courses of increasing and decreasing steps of the magnetic field applied on the MR device. FIG. 19 shows the resistance variation by the magnetic field applied in the orthogonal direction on a MR device, and the hard lined and the dotted lined curves show such a resistance variation in the cases of during the decreasing and increasing magnetic field intensity, respectively. Both the wiring leads 73 as well as 83 are formed without taking consideration of such characteristics. Therefore, in a position detection of higher precision, the measuring error is inevitable.
Further, the extent of the resolution is around 1/4 of the magnet width at best. Also, in the case of interpolating the output curve, the position information is affected by a degree of sine wave distortion. As a result, by the conventional method, the measuring error is inevitable in a position detection of higher precision.